Aim: Histamine is a chemical mediator involved in postoperative periapical inflammation and pain by increasing vascular permeability, inflammatory edema, and lowering pain threshold. The purpose of this study was to investigate the effect of astemizole (H1 receptor antagonist) on postoperative periapical inflammation.

Method & Material: In this clinical trail, twelve healthy mature cats were divided into two experimental groups. First group (6 animals) received 10mg astemizole (antihistamine drug) orally one hour before root canal treatment and second group (six animals) received no medication. Animals were sacrificed at 6, 12, and 24 hours after root canal treatment. Six-micron sections were prepared and stained with H & E and examined under light microscope.

Results: Comparing the two experimental groups using the wilcoxon sign test showed that edema and vasodilation in astemizole group were significantly (P<0.05) less than without astemizole group at 6 hours after treatment but there was no statistical difference between two groups at 12, 24 hours after treatment.

Conclusions: It was concluded that astemizole might be helpful in postoperative pain control by reducing edema and vasodilation.